1. Field of the Invention
This invention relates to a supervised security alarm system having audio monitoring capabilities, and more particularly to an alarm system wherein one or more remote stations to be protected can communicate with a central monitoring station via a two-wire telephone line to provide line supervision, alarm condition recognition and indication, and audio monitoring capabilities.
2. Description of the Prior Art
There are many types of security alarm systems wherein a remote station under surveillance is interconnected via transmission lines to a central monitoring station to allow an operator at the central station to monitor for various alarm conditions. Most of these systems simply trigger an audible or visual alarm when a particular alarm condition, such as the breaking of a perimeter protection loop at the remote location has occured. Very few of the systems of the prior art provide audio monitoring capabilities at the remote location and most of those that do, utilize sounds or sonic vibrations at the remote location to trigger an alarm indication at the central station rather than monitoring the actual sounds generated at the remote location itself.
Most of the central stations of the prior art utilize a plurality of relay means to detect changes in conditions at the remote station and to generate the alarm indication. Many of these relays are relatively slow-acting, involve moving parts which are mechanically unreliable, and require relatively large power supplies for operation.
The few sound monitoring systems which do exist, generally rely on sound monitoring alone and are not generally used in combination with other security alarm systems. Furthermore, they are usually continuously powered resulting in an unnecessary invasion of the privacy of the person whose remote station is being protected. If privacy is desired and the power is turned off, the sound monitoring capability, and generally the entire alarm detection capability of the system is lost.
The few sound monitoring systems of the prior art are further plagued with the problems that they are easily triggered by normal environmental noise or random noise generated in the vicinity of the remote location. Tolerance settings and threshold levels become critical, effect the operation of the system and often lead to all-too-numerous false alarms and an eventual failure of confidence in the system.
In general, the systems of the prior art are relatively simple and can be defeated by a relatively unsophisticated burglar or intruder. Furthermore, the systems are prone to false alarms and to mechanical failures. Additionally, the systems are relatively expensive and difficult to maintain and have a relatively slow response time while consuming considerable quantities of power. The central monitoring stations are especialy susceptible to damage from surge currents and voltage spikes such as may be generated by system irregularities, lightening, and the like which can render the central monitoring station inoperative and cause damage to the expensive monitoring equipment.
The present invention overcomes these disadvantages of the prior art by providing a low-cost, easy-to-operate maintenance-free, highly reliable central monitoring station which is virtually isolated from transmission line irregularities and which provides (1) line supervision capability, (2) alarm condition detection and indication capability, and (3) sound monitoring capabilities which allows the operator at the central station to monitor the actual sounds originating from he remote location. The present invention also provides a relatively sophisticated security alarm system at the remote location which incorporates (1) a sound monitoring system, (2) normal closed loop perimeter protection capability, (3) the monitoring of any number of distinct and different types of alarm conditions such as hold-up, fire, and the like, (4) signal injection capability for superimposing different and distinct audio tones onto the DC transmission signal to allow the operator at the central station to differentiate between various types of emergency conditions, and (5) one or two way coding key capability to allow either the operator at the central station, a person at the remote location, or both, to send coded signals to one another.